One of my local ISP offers 10Gbps broadband for cheap. What is the cheapest router setup one can get with a 10Gpbs wan and lan port? WiFi and switching hardware is optional.
In the long run a PC will probably more expensive though, because it uses more electricity than a specialized embedded device. A specialized router is probably worth it if you stick with it for a while, at 10G there is little reason to upgrade.
Unless you go with a pc with 10gig interfaces my suggestion would be a mikrotik ccr2004-16g-2s and a couple of 10gig sfp+ interfaces. It comes with the added bonus of passive cooling and more software options than most of us knows what to do with. Even has WLC capabilities.
Zyxel AX7501 has one sfp+ 10G wan and one rj45 10G lan port.
Should be around 350$ if you can buy it where you live. I work at an ISP am that's the device we're giving to our customers for 10G.
In general I don't think you'll get much lower than that even if you build your own router.